Occurrence Of Basalts Research Articles

Flood basalt origin for Earth’s largest Paleoarchean banded iron formation

A genetic relationship between banded iron formations (BIFs) and flood basalts has long been assumed based on their close temporal and spatial association. However, proving causation has remained difficult given the absence of geochemical evidence, leading some authors to speculate that BIFs are not primary deposits. In this study, we examined the entire lithological succession of the western Iron Ore Group from the Singhbhum craton, which includes Earth’s largest, economic-grade BIF of Paleoarchean age. We show the Lower Lava flood basalts to plot on a Sm-Nd isochron with the conformably overlying Lower Shale, suggesting a 3.42 ± 0.10 Ga depositional model age for this iron-rich (average 18.26 Fe2O3 wt%, n = 7) clastic unit, implying it may be a precursor to the conformably overlying 3.39 ± 0.02 Ga BIF. This is the first occurrence of flood basalts being genetically linked to ferruginous shale on a Sm-Nd isochron and may support the hypothesis BIFs are products of secondary replacements of ferruginous clastic precursors. We assign a volcanogenic origin to this Algoma-type BIF and suggest its abnormally large size is related to the unroofing and weathering of the Lower Lava flood basalts during a major crustal formation event.

Early Paleogene alkaline magmatism in the Subandean Ranges of Jujuy Province

The Zapla Range (Jujuy province) represents the southern edge of the central Andean Subandean Ranges in Argentina. In the northernmost area of the Zapla Range, basaltic rocks are interbedded in the Mealla Formation. These volcanic units form part of the post-rift magmatism of the Salta rift basin, which mainly focused on the Lomas de Olmedo sub-basin to the east. In this paper, six of these basaltic occurrences are characterized on the basis of new geological, petrographic and geochemical data, in order to explore their emplacement, petrogenesis and possible tectonic setting during rift evolution. Peperites in the upper contact of sills evidence the interaction between basalts and unconsolidated or poorly consolidated wet sediments. This implies that there was concomitance between sedimentation and subsurface magma emplacement, suggesting a Paleocene age for the magmatic event according to zircon U-Pb data available for Mealla Formation rocks. Other basaltic units show hipocrystalline textures and lack peperites at the top, which points to their extrusive nature. Additionally, some of these lava flows are spatially associated with pyroclastic deposits that reveal the occurrence of explosive hydrovolcanic eruptions. The study rocks are classified as alkaline basalts and basanites with high Mg# and Ni- and Cr-rich compositions, close to primitive magmas. They show affinity with continental intraplate settings. The analyses of Ba/Nb, Nb/La, (La/Yb)N, (La/Sm)N, (Gd/Yb)N and (Dy/Yb)N ratios, as well as normalized trace and rare earth element patterns, suggests a genesis by low degrees of partial melting of a residual garnet-bearing mantle source. Therefore, basalts from this part of the Subandean Ranges evidence a magmatic episode probably linked to an extensional reactivation stage of the Salta Group rift system in the Lomas de Olmedo depocenter during the Paleocene (∼60 Ma).

Magnetic response contrast of Oligo-Miocene basaltic magmatism and structural control during its ascent at the northern tip of the Red Sea Rift

Oligo-Miocene volcanic activity has occurred along the northern Egyptian margin of the Red Sea Rift system, leading to the creation of small-volume monogenetic volcanoes. This research delves into the morphology and setting of these rift-related volcanics by combining aeromagnetic data, remote sensing, and field verification. Basaltic exposures in the study area were mapped using False Color Composite (FCC) and band ratios in Landsat 8 OLI data. In a map view, these mappable volcanic rocks appear as discrete semi-circular basaltic bodies and as discontinuous dikes. The basaltic bodies are aligned along ENE and NW-oriented zones, with most dikes orientated NW-SE. Multiple isolated high magnetic zones with NE and NW orientations were identified. Most basaltic outcrops coincide with high magnetic zones except for Gabal Abu Treifiya along the Cairo-Suez Road. The integration and combination of three layers of surface and subsurface structure patterns reveal an array of variance. The dominant trend of surface and deeper structural lineaments is NW-SE and NE-SW respectively. In contrast, shallow structural lineaments have NW-SE, NE-SW, E-W, and N–S trends. Long linear grabens result from the pairing of NW-SE normal faults. The basaltic occurrences such as Qattamiya and Abu Treifiya are located at the intersection of the NW-SE and E-W normal faults along the NW-SE linear grabens. The field investigation at both locations reveals a polyphase magmatic feeding system manifesting in the surface today as coherent and volcaniclastic rock units. This suggests a diverse range of eruptive styles through a complex feeder system that involves more than one volcanic cone. Though the two areas have good basaltic exposures, yet they have different magnetic responses. The severe alteration of Abu Treifiya's volcanic rocks into nonmagnetic minerals assemblage of zeolite, smectite, calcite, and hydrolysis of basaltic glass diminishes the expected high magnetic anomaly. This study contributes to our knowledge of Rift-related volcanism and the factors influencing the distribution of these volcanic materials.

Alkaline picritic volcanism on northern Ellesmere Island associated with initial rifting of the Sverdrup Basin, Canadian Arctic

The Caledonian and Ellesmerian orogenies were followed by extension and development of intracontinental rift basins since the late Paleozoic, as represented by the Sverdrup Basin along the Canadian Arctic Islands. The initial rifting was accompanied by pulses of volcanic activity during the Carboniferous and Permian. A new occurrence of mafic volcanic rocks, the “Taconite volcanics” (informal name), was discovered on northern Ellesmere Island between the head of Ayles Fiord and M’Clintock Inlet. The mainly alkali-picritic lavas are exposed within the central part of the Pearya Terrane as an outcrop in faulted contact with upper Carboniferous red beds of the Canyon Fiord Formation. The contact to the Ordovician island-arc volcanic rocks of the Pearya Terrane is unclear. The outcrop is characterized by a small circular magnetic anomaly. Ar–Ar whole-rock geochronology on the volcanic rocks yielded an age of 290 ± 19 Ma suggesting emplacement of the lavas during the early Permian. Whole rock geochemical analyses for eight samples revealed a geochemical affinity to ocean island basalt (OIB) and indicate variable mixing of low-degree melts in the fields of garnet and spinel peridotite (∼80–90 km depth). The involvement of a metasomatized subcontinental lithospheric mantle is indicated by high Pb, Nb, and Ta concentrations. Geochemical differences (as enrichments in Ti, Nb, Zr, and the light rare earth elements (REE)) to the known Carboniferous and Permian spilitic altered basalt occurrences of northwestern Ellesmere and northern Axel Heiberg islands are probably based on differences in the mantle source. The Sr isotope ratios of the Taconite volcanics are primitive ((87Sr/86Sr)t: 0.7037–0.7042) and its Nd isotope ratios are moderately depleted (εNd(t): +2.20 to +2.85) in contrast to the enriched εNd(t) values of the Permian Esayoo formation.

The discovery of the Siberian Traps Large Igneous Province in 1870s, and the leading role of Aleksander Czekanowski

Flood basalts (traps) and large igneous provinces (LIPs) are essential topics in modern geology. However, little is known about the stages of recognition of particular LIPs, including a giant continental LIP in Siberia. Even though basalt occurrences were reported from the region from the end of the 18th century (e.g., by Kozitsky, Middendorf, Schmidt and Lopatin), the first ample data were provided by the Polish geologist Aleksander Czekanowski (1833–1876). He was educated at the Universities of Kiev and Dorpat but exiled to Siberia for participating in the Polish January Uprising in 1863. Czekanowski organised three expeditions to poorly known parts of Central Siberia, which were initiated by exploring the Lower Tunguska River Basin. In the summer of 1873, he discovered many exposures of basalts and related tuffs. After an expedition to the Olenek River region in 1874, he was aware of a vast extent of basalt sequences and the critical impact of the “powerful volcanic floods” that immensely transformed the uplifted continent. Czekanowski emphasised “the discovery of previously unknown areas of igneous rocks of so large an extent that it exceeds the size of any other of its kind.” Studies of the volcanic series developed towards the end of the century, as summarised in an influential treatise from 1901 by the great Austrian geologist Eduard Suess. Czekanowski, distinguished in Seuss's monograph, is occasionally appreciated as the discoverer of the Siberian Traps but should also be adequately honoured in the neocatastrophism-based geology of the 21st century.

Evidence for the essential role of CO2 in the volcanism of the waning Caroline mantle plume

Deep-rooted mantle plumes provide a link for carbon between Earth’s interior and its surface. Low-degree partial melting of CO2-bearing mantle is predicted to produce carbonated melts in the deep mantle, but carbonated melts are rarely found in modern oceanic settings, and the role of primary carbonated silicate melts in mantle-plume volcanism remains unclear. Because low-degree mantle melting tends to generate CO2-rich melts, we studied a suite of volcanic rocks from Pohnpei Island formed by the waning Caroline mantle plume based on whole-rock major- and trace-element, Sr–Nd–Pb–Hf isotopic, and high-precision in situ mineral compositions. We demonstrate that the mineralogy and geochemistry of these volcanic rocks are consistent with those of high-MgO primary carbonated silicate melts, e.g., they have the most SiO2-depleted and CaO-enriched compositions reported for ocean-island basalts, they are enriched in rare-earth elements and have negative anomalies of high-field-strength-elements. The Sr–Nd–Pb–Hf isotopic compositions of these volcanic rocks exhibit mixing between the Pacific-type depleted mantle and an enriched-mantle II (EM2)-like source. We found that primary carbonated silicate melts were modified to become typical ocean-island alkali basalts through reactive evolution and CO2 degassing during magma ascent in the lithosphere, and also identified a closed-system stage of evolution after these melts reached equilibrium with the lithospheric mantle. We suggest that low-degree melting of a waning mantle plume facilitates carbonated-silicate-melt volcanism, with the carbonated silicate melts being converted to typical alkali ocean-island basalt in the overlying thick lithospheric mantle. The anomalously low olivine Ca and Mn contents and the Ca partition coefficient between olivine and melt suggest that the primary magmas had a CO2 content of 11–15 wt.%. These volcanic rocks also contain Hawaii-like high-Ni olivines, which we interpret as reflecting a pyroxenite-rich and olivine-poor mantle source. The proposed reactive evolution of carbonated melts in the lithospheric mantle tends to cause loss of CO2 in the deep mantle and regulates the fate of carbonated melts, which may explain the widespread occurrence of alkali basalts and the rarity of carbonated melts in intra-oceanic-plate settings.

Isolated alkaline basalt occurrences in the northern Spessart, Germany: Outposts of the Early Miocene Vogelsberg shield volcano?

Four isolated occurrences of Tertiary volcanic rocks in the northern Spessart at Beilstein, Hoher Berg, Madstein and Kasselgrund are relics of volcanic vents or dikes. They display alkaline basalts (s. l.) with mainly trachybasaltic composition, which, from normative mineral contents, may be designated as nepheline-bearing alkali-olivine basalts and basanites. In part, centimetre-sized xenoliths of spinel lherzolite occur. According to Ar-Ar dating, the alkaline basalts (s. l.) from Kasselgrund have erupted at 18.1 ± 0.3 or 19.3 ± 0.4 Ma, those of Hoher Berg between c. 18 and c. 21 Ma. These ages correspond to the Vogelsberg eruption stage I. A slightly younger Ar-Ar age of 16.8 ± 0.3 Ma was recorded for the Beilstein basalt, which is in chronological accordance to the turn of Vogelsberg eruption stages II and III. Samples of all four occurrences reveal major and trace element compositions, which are different from those of the Vogelsberg basalts. Compositions of basalts of the stage III from Vogelsberg coincide most with the Spessart basalts. This signals a special position of the northern Spessart volcanic rocks either as a discrete spatial part of the Vogelsberg volcanic suite or as smaller, independent eruption centres.

Coupled supercontinent–mantle plume events evidenced by oceanic plume record

AbstractThe most dominant features in the present-day lower mantle are the two antipodal African and Pacific large low-shear-velocity provinces (LLSVPs). How and when these two structures formed, and whether they are fixed and long lived through Earth history or dynamic and linked to the supercontinent cycles, remain first-order geodynamic questions. Hotspots and large igneous provinces (LIPs) are mostly generated above LLSVPs, and it is widely accepted that the African LLSVP existed by at least ca. 200 Ma beneath the supercontinent Pangea. Whereas the continental LIP record has been used to decipher the spatial and temporal variations of plume activity under the continents, plume records of the oceanic realm before ca. 170 Ma are mostly missing due to oceanic subduction. Here, we present the first compilation of an Oceanic Large Igneous Provinces database (O-LIPdb), which represents the preserved oceanic LIP and oceanic island basalt occurrences preserved in ophiolites. Using this database, we are able to reconstruct and compare the record of mantle plume activity in both the continental and oceanic realms for the past 2 b.y., spanning three supercontinent cycles. Time-series analysis reveals hints of similar cyclicity of the plume activity in the continent and oceanic realms, both exhibiting a periodicity of ∼500 m.y. that is comparable to the supercontinent cycle, albeit with a slight phase delay. Our results argue for dynamic LLSVPs where the supercontinent cycle and global subduction geometry control the formation and locations of the plumes.

Tokoro Belt (NE Hokkaido): an exhumed, Jurassic – Early Cretaceous seamount in the Late Cretaceous accretionary prism of northern Japan

AbstractThe Tokoro Belt exposed in NE Hokkaido (Japan) represents part of a Late Cretaceous accretionary complex, which includes variously metamorphosed volcanic rocks that are interbedded with chert, lenticular limestone and some fore-arc sedimentary rocks. The Tokoro Belt is notably different from other Late Cretaceous accretionary complexes around the Pacific Rim because of widespread occurrence of basalts and volcaniclastic rocks in it. The Nikoro Group, characterized by widespread occurrence of volcanic rocks, is divided into western, eastern and southern sections based on the internal structure, geochemical affinities and metamorphic grades of their volcanic lithologies. OIB (ocean island basalt)-type volcanic rocks with low-grade metamorphic overprint predominate in the western and southern sections, whereas MORB (mid-ocean ridge basalt)- and OIA (ocean island alkaline basalt)-type rocks in the eastern section with partly high-pressure metamorphism make up the northern part of the eastern section. Trace element patterns display transitional trends from MORB to OIA geochemical affinities. OIB-type rocks display trace element characteristics similar to those of shield volcano lavas on Hawaii, rather than small and mainly alkaline, Polynesian hotspot lavas; furthermore, they show significant HREE (heavy rare earth element) enrichment probably caused by plume–ridge interaction. Widespread OIBs in the Tokoro Belt represents tectonic slices of a large (>80 km wide) Hawaiian-style, seamount shield volcano on the Izanagi oceanic plate that was accreted into the continental margin of Far East Asia in the Late Cretaceous.

Extension of flood basalt on the northwestern continental margin of India

The Deccan Continental Flood Basalt (DCFB) in central western India is a large igneous province that covers almost one-sixth area of India. It is erupted in less than 1 million years during the magnetic chron 29 r ( $$\sim $$ 65.6–64.8 Ma) through the Cretaceous–Tertiary boundary. It is believed that nearly an equivalent area of flood basalt, adjoining the DCFB, is submerged on the northwestern continental margin of India (NWCMI). Onshore information on the DCFB is widely reported in geological and geophysical studies. However, knowledge on the offshore extent of flood basalt is poorly known because of paucity of marine geophysical data and lack of an appropriate approach to determine the flood basalt. To fill this gap in knowledge, P-wave velocity in flood basalt as a proxy, drilled wells results, and published seaward dipping reflectors are used to delineate flood basalt extent on the NWCMI. The results of the study reveal that P-wave velocity in the flood basalt varies from 4.1 to 5.2 km/s. Apart from some isolated basement-high features, it is found that flood basalt lies below sediment and carpets the entire NWCMI extending up to the Laxmi–Laccadive ridges. The depth of occurrence of flood basalt ranges from 800 to 7400 m, with a maximum thickness of $$\sim $$ 3900 m in the eastern part of the Laxmi basin. The article presents preliminary results about the extension of flood basalt on the western continental margin of India which may be useful for researchers and Indian oil industries planning exploration activities in Mesozoic sediment of the margin for hydrocarbon prospects.

Mineralogy and geochemistry of recent alluvial sediments from the Ngaye River watershed, northern Cameroon: Implications for the surface processes and Au-PGE distribution

In this work, six vertical sequences were selected to understand the petrological features of recent alluvial sediments as well as Au-PGE distribution in heavy mineral concentrates from two terraces of Ngaye River watershed in Northern Cameroon, using mineralogical (XRD) and geochemical (XRF and ICP-MS) instruments. The Ngaye River watershed is located in the tropical region with contrasted seasons. The lithotypes are mainly made up of gneisses, granites, and amphibolites with different textures. Granites contain high average contents (n = 7) in SiO2 (69.54 wt.%), Al2O3 (15.21 wt.%), Na2O (4.39 wt.%) and K2O (2.00 wt.%) contrary to gneisses with moderate average contents (n = 7) in SiO2 (64.32 wt.%), Al2O3 (15.75 wt.%), Fe2O3 (5.03 wt.%), CaO (4.45 wt.%), and MgO (2.79 wt.%). Several major element contents are higher in gneisses than amphibolites except for Fe2O3 (11.38 wt.%), CaO (8.63 wt.%), and MgO (6.41 wt.%). Amphibolites (n = 7) are more enriched in Cr, V, Ni, Zn, and Co whereas granites exceed in Ba, Sr, and Rb. Ngaye rocks have moderate REE content (75–122 ppm) with variable LREE/HREE values (5.88–18.83). The alluvial sediments are mainly clayey sandy and are made up of abundant quartz associated to smectite, kaolinite, feldspars, illite, and accessory rutile, amphibole, olivine, pyroxene, ilmenite, and goethite. The occurrence of olivine in one the sequences approves the occurrence of basalts in the watershed. The SiO2 contents range from 55 to 83.41 wt.%, the contents of this oxide show an increasing tendency while those of Al2O3, Fe2O3, MgO, TiO2, and LOI decrease. Several oxides are negatively correlated with SiO2; this confirms the high proportion of silicate minerals and different weathering of source rocks (ICV ∼0.77–0.96; CIA ∼59–80%; PIA ∼64–83%). These sediments are also highlighted by high contents in Ba, Zn, and Sr, and moderate contents in transition trace metals (Cr, V, Ni, and Zr) which confirm the felsic nature of the source rocks. Meanwhile, selected trace elements correlate negatively with SiO2 and explain the low contents in trace elements. The total REE content is twofold high compared to the lithotypes but the LREE/HREE values show similar trend like those of the rocks. The spider diagrams display (i) homogeneity of patterns; (ii) negative anomalies in U, Nb, and P; (iii) and slight positive Zr anomalies. Chondrite-normalized REE patterns exhibit negative Eu anomalies (Eu/Eu∗ = 0.56–0.89) contrary to the PAAS normalization (Eu/Eu∗ = 1.25–1.37). The total Au-PGE contents are very low (1.49–3.94 ppb) in the heavy mineral concentrates contrary to slightly higher contents in Ngaye amphibolites (8.43 ppb). Despite the low contents, the heavy mineral concentrates are richer in PPGE, particularly in Pd and Pt. This geochemical survey has revealed that further alluvial Au-PGE exploration may not be necessary.

A tale of two rift shoulders, and two ice masses: the Cryogenian glaciated margin of Death Valley, California

Abstract The Death Valley area of California, USA, exposes an outstanding record of a Neoproterozoic (Cryogenian) glaciated margin: the Kingston Peak Formation. Despite the quality of the exposure, however, the outcrops of glaciogenic strata are fragmentary, forming isolated, laterally offset outcrop belts at the western extremity of the Basin and Range province. Excellent evidence for glacially modulated sedimentation includes (1) ice-rafted dropstones in most ranges, (2) thick diamictites bearing a variety of exotic (extrabasinal) clasts, (3) striated clasts and (4) local occurrences of glacitectonic deformation structures at the basin margins. In tandem with this, there is a distinct signature of slope collapse processes in many ranges, including (1) up to kilometre-scale olistoliths, (2) extensional growth fault arrays, (3) dramatic proximal-distal thickness changes and (4) basalt occurrences. New sedimentological observations reinforce long-held views of rifting superimposed on glaciation (or vice versa), with both processes contributing to a complex record whereby rift and glacial processes vie for stratigraphic supremacy. We consider that a mechanism of diamictite accumulation in a series of rift-shoulder minibasins produced greatly contrasting successions across the Death Valley area, under the incontrovertible influence of hinterland ice sheets.

Continental intraplate volcanism in the Sinai subplate: The Oligo-Miocene basalts of the Gulf of Suez rift

Three major Oligo-Miocene basaltic occurrences in Sinai, along the eastern side of the Gulf of Suez rift (Wadi Tayiba, Wadi Matulla and Gabal Abu Durba), have been chosen to reveal the geochemistry and petrogenesis of the magmatism associated with this newly-formed continental rift. They include rift-related basaltic/doleritic dikes and sills and isolated basaltic lava flows, belonging to a prolonged persistent magmatic event occurred around 20 Ma, that crosscut older Phanerozoic strata and Precambrian basement. The Gulf of Suez rift (GSR) volcanics show confined range of SiO2 contents (47.51–52.25 wt%) and Mg# values (31–48) and consist of alkaline basalts, hawaiites, mugearites and transitional tholeiitic basalts. All of the investigated basaltic samples possess explicit Fe-enrichment and sodic nature and are silica-oversaturated (quartz-normative) and metaluminous (diopside-normative and have NK/A<1 and A/CNK <1). The compatible transition elements Ni (9–69 ppm), Cr (14–123 ppm), V(237–453 ppm), Sc (19–43 ppm) are generally depleted relative to primitive lavas, consistent with the evolved nature of these basalts (e.g. low Mg# (<68) and silica-oversaturation). Conversely, these basalts show strong enrichment of most of the incompatible LIL and HFS elements relative to primitive mantle, analogous to ocean island basalts. The GSR basalts have suffered minimal degree of crustal contamination, which was likely a minor contribution from the lower crust that superimposed on crystal fractionation. The Nb/Ta and Zr/Hf ratios of the studied basalts are comparable to those of OIB and enriched MORB and are suggestive of their derivation from mantle source. The GSR basalts were most probably derived via partial melting of quite deep asthenosphere (>70 km) amphibole-bearing garnet peridotite mantle source.

Early Paleozoic subduction initiation volcanism of the Iwatsubodani Formation, Hida Gaien belt, Southwest Japan

In placing Japanese tectonics in an Asian context, variation in the Paleozoic geological environment is a significant issue. This paper investigates the geochemistry of the lower Paleozoic basalt formation (Iwatsubodani Formation) in the Hida Gaien belt, Japan, to consider its tectonic setting. This formation includes the following two types of rock in ascending order: basalt A with sub-ophitic texture and basalt B with porphyritic texture. Basalt A has a high and uniform FeO*/MgO ratio, moderate TiO2, high V, and low Ti/V. The HFSE and REE are nearly the same as those in MORB, and all the data points to basalt A being the “MORB-like fore-arc tholeiitic basalt (FAB)” reported, for example, from the Izu–Bonin–Mariana arc. By contrast, basalt B has a low FeO*/MgO ratio, low TiO2, and low V and Ti/V. It has an LREE-enriched trend and a distinct negative Nb anomaly in the MORB-normalized multi-element pattern and a moderately high LREE/HREE. All these factors suggest that basalt B is calc-alkaline basalt. It is known that FAB is erupted at the earliest stage of arc formation—namely, subduction initiation—and that boninitic/tholeiitic/calc-alkaline volcanism follows at the supra-subduction zone (SSZ). Thus, the occurrence of basalts A (FAB) and B (calc-alkaline rock) is strong evidence of early Paleozoic arc-formation initiation at an SSZ. Evidence for an early Paleozoic SSZ arc is also recognized from the Oeyama, Hayachine-Miyamori, and Sergeevka ophiolites. Hence, both these ophiolites and the Iwatsubodani Formation probably coexisted in a primitive SSZ system in the early Paleozoic.

Tectonic implications of Early Miocene OIB magmatism in a near-trench setting: The Outer Zone of SW Japan and the northernmost Ryukyu Islands

The Outer Zone of the SW Japan and northernmost Ryukyu arcs was affected by intense igneous activity during the Miocene, characterized by MORB-like basalts, alkaline basalts, and S-type (with subordinate I-type) felsic to intermediate volcano-plutonic complexes. These igneous rocks are inferred to be the products of near-trench magmatism. Early Miocene (∼18Ma) alkaline basalt dikes from the Shingu–Otoyo area in central northern Shikoku, and an alkaline lamprophyre dike from Tanegashima, one of the northernmost Ryukyu Islands, pre-date the Middle Miocene felsic to intermediate igneous rocks. The basalts and lamprophyre have compositions of basanite, basalt, trachybasalt and phonotephrite. They are characterized by elevated large-ion lithophile elements (LILEs; e.g., Sr, Ba, and Th) and high concentrations of high-field strength elements (HFSEs; e.g., TiO2, Nb, and Zr). The geochemical signatures of the basalts and lamprophyre suggest an ocean island basalt-type (OIB-type) mantle source. The occurrence of alkaline basalts and lamprophyre with OIB-type, intraplate geochemical signatures in a near-trench setting is unusual with regard to plate tectonic processes.We propose that trench-ward motion of the overriding plate during the period around the Early Miocene resulted in a shallowly dipping slab, and interplate coupling between the subducting Philippine Sea Plate (PSP) and the overlying crust beneath most of the Outer Zone in the western part of SW Japan and the northernmost Ryukyu Islands. The OIB-type magmatism in the near-trench environment is most plausibly explained by the upwelling of asthenospheric material from beneath the subducting slab, which migrated through fractures and/or tears in the slab. We envisage two possible scenarios for the formation of these fractures or tears: (1) the shallowing dip angle of the subducted PSP resulted in concave-upwards flexure of the slab, generating fractures in the flexed region; and (2) differential motion within the subducting slab induced slab tearing and segmentation.

Olivine-phyric basalt in the Mesoproterozoic Gawler silicic large igneous province, South Australia: Examples at the Olympic Dam Iron Oxide Cu–U–Au–Ag deposit and other localities

The felsic-dominant Gawler Range Volcanics and cogenetic Hiltaba Suite granitoids constitute the ca. 1590Ma Gawler silicic large igneous province in the Gawler Craton, South Australia. The province includes minor occurrences of olivine-phyric basalt at Kokatha and Mount Gunson. In this study, we describe additional olivine-phyric basalts and dykes intersected by drill holes at the Olympic Dam Iron Oxide Cu–U–Au–Ag deposit and the Wirrda Well Cu–Au prospect. Laser Ablation Inductively Coupled Plasma Mass Spectrometry U–Pb dating results (un-anchored) of apatite in the basalts and dykes at Mount Gunson (1576±33Ma), Wirrda Well (1596±17Ma), and Olympic Dam (1621±20Ma) confirm their effective temporal correlation with the ca. 1590Ma Gawler silicic large igneous province.Compositions of Cr-spinel inclusions enclosed in former olivine phenocrysts (forsterite number >80) in basalts at Mount Gunson, Wirrda Well and Olympic Dam imply derivation from a heterogeneous mantle source that may have been modified by subduction. Least-altered olivine-phyric basalts at Mount Gunson and Kokatha are characterized by negative Nb and Ta anomalies, which are typical of arc basalts but are also common in back-arc basin basalts, in accordance with the result suggested by the Cr-spinel source indicator. The mafic components of the Gawler Range Volcanics generally have higher Zr contents and Zr/TiO2 ratios than those of high-Mg basalts and picrites produced in variable tectonic settings worldwide, possibly reflecting continental crustal components involved in their mantle source.High abundances (typically ∼20vol.%) of former olivine phenocrysts in the basalt at Olympic Dam imply high-Mg whole-rock compositions and a high temperature of the primary magma. This result is consistent with previous models in which the heat flux from mantle magmas caused large-scale partial melting of crustal rocks that subsequently gave rise to silicic magmas erupted and intruded to form the Gawler silicic large igneous province.

Hydrothermal processes related to Triassic and Jurassic submarine basaltic complexes in northeastern Hungary and in the Dinarides and Hellenides

Comparative studies on hydrothermal alteration of submarine peperitic basalt occurrences related to the Triassic early rifting of the Neotethys were carried out in various parts of the Dinarides and Hellenides. The study areas included the displaced fragments of the Dinarides in the Darno Unit, NE Hungary, the Kalnik Mts in Croatia and the Vares-Smreka area in Bosnia and Herzegovina. In the Hellenides, similar environments were studied in the Stragopetra Mts., Greece. Jurassic pillow basalts formed in a back-arc-basin of the Neotethys were also studied (in the Szarvaskő Unit, NE Hungary, which also represents a displaced unit of Dinaridic origin). Within the submarine basaltic lava flows, six volcanic facies were distinguished. The hydrothermal alteration was characterized according to those facies. The first process was the albitization of the rock-forming plagioclase at ~300°C temperature in all localities. During the higher temperature stage of the subsequent cooling, chloritization in the ground mass is typical for all types of basalts, however chlorite and rarely quartz formed in the fractures and amygdales of the Triassic basalts, while chlorite, quartz and prehnite precipitated in the fractures of the Jurassic rocks. At lower temperatures of this cooling-related process, calcite is a common mineral filling up the larger amygdales, jig-saw type fractures and other open spaces, but some epidote, pumpellyite, prehnite and laumontite also occur in the Triassic basalts. The late stage alteration (happened at the lowest temperature) is characterized by argillitization at every locality. The observed hydrothermal alteration patterns also show slight differences according to the volcanic facies as a function of the distal/proximal setting in relation to the eruptive centers and the presence/absence of water-saturated and unconsolidated carbonate or siliciclastic sediments at the time of the emplacement of lava flows. The study revealed that the most important factors influencing mineralogy and zoning of hydrothermal alteration in these short living local hydrothermal systems are the rapid cooling of the hydrothermal fluid, the dominance of the not much evolved seawater as the source of hydrothermal fluid and the local, i.e. effective water/rock ratio, determined by the degree of fracturing in the rock. The mineralogical-textural peculiarities of the highly localized hydrothermal fluid/rock interaction in the studied submarine sea-mount type volcanoes are clearly different from the products of the large-scaled hydrothermal processes occurring at mid-oceanic ridges. Recognition of these differences is important in the evaluation of ore potential of Neotethyan realm or other areas with occurrences of submarine basaltic units

Small-scale lunar farside volcanism

We identified and mapped 28 mare basalt occurrences, between the Australe and South Pole-Aitken basins on the southern lunar farside, and determined their absolute model ages (AMAs) by performing crater size–frequency distribution (CSFD) measurements. Our study area can be subdivided into seven major mare basalt occurrences in and around Bolyai, Roche V, Rosseland, Pauli, and Roche craters, south of both Rosseland and Coblentz craters, as well as mare patches between Eötvös and Roche craters. The AMAs of the mare basalts in and around Bolyai crater range from 2.1Ga to 3.5Ga (two units), varying drastically within short distances. The mare patches south of Coblentz crater contain nine units that have AMAs ranging from 2.1Ga to 3.8Ga. The mare basalts in Roche V crater show an AMA of 2.2Ga. We mapped seven volcanic units in Rosseland crater and derived AMAs for five of these units. The mare basalts in Rosseland crater show the youngest AMAs found in our study area, ranging from 1.5Ga to 2.9Ga. The mare basalt occurrence south of Rosseland crater shows significantly older ages with an AMA of 3.3Ga. The mare basalts in Pauli and Roche craters show AMAs from 1.7Ga to 3.1Ga. The mare patches between Eötvös and Roche craters show a similar range of AMAs from 1.9Ga to 3.1Ga. The AMAs of the mare basalts in our study show that the lunar farside was volcanically active for nearly as long as the lunar nearside (1.2Ga ago), or at least longer than previously thought (2.5Ga ago) and predicted by models of the ascent and eruption of lunar basalts. In addition, we calculated thicknesses and volumes of the investigated mare basalts. With thicknesses between ∼21m and ∼172m and volumes from ∼0.1km3 to ∼379km3, the mare basalts in our study area show a wide range of dimensions, similar to other mare basalts of the lunar near- and farsides.

Petrogenesis of basalt–high-Mg andesite–adakite in the Neoarchean Veligallu greenstone terrane: Geochemical evidence for a rifted back-arc crust in the eastern Dharwar craton, India

The ∼2.7Ga Veligallu greenstone terrane in the eastern Dharwar craton is one of the least studied Neoarchean greenstone belts in India. To our knowledge, there is no published dataset that provides a detailed account of the petrogenetic processes, which culminated into the evolution of this greenstone belt. This paper presents a comprehensive account of field, petrography and geochemistry of the basalt–high-Mg andesite–adakitic volcanic rock association in the Veligallu greenstone belt.The basalts are tholeiitic in nature and are characterized by primitive geochemical compositions with ∼49wt% SiO2, ∼9wt% MgO, Mg# 54, Cr∼197ppm, Ni∼122ppm, Y∼27ppm, Yb∼2.7ppm; Zr/Y<3 and Ce/Yb<5. They do not plot on a mixing line between Mesoarchean TTG and N-MORB, therefore, inconsistent with any interaction with the upper continental crust. Accordingly, they are interpreted to have erupted in an oceanic setting. On the basis of Nb/Th ratio the Veligallu basalts are divided onto two groups. The basalts with Nb/Th>8 display systematically flat to light-REE depleted chondrite normalized REE patterns (LaN/YbN=0.76–1.0), in contrast to the samples with Nb/Th<8, which display slight enrichments in light-REE (LaN/YbN=1.0–1.6). The depleted basalts are characterized by primitive mantle normalized Nb concentrations lower than La but equal to Th, in contrast to the enriched basalts, which are characterized by slight depletions of Nb relative to both La and Th. Collectively, as a group, they display Nb/Yb ratio higher than the N-MORB. The enrichment in Nb is attributed to moderate to low degree (∼10%–4%) partial melting of a depleted MORB source. The trace, REE and their interelement ratios in the Veligallu basalts exhibit transitional characteristics between the average N-MORB and the typical Phanerozoic island arc basalts. Overall, the geochemical characteristics of the Veligallu basalts are identical to basalts erupted in the Phanerozoic back-arc basins.The Veligallu high-Mg andesites are orthopyroxene-bearing calc-alkaline andesites and are characterized by moderate SiO2 (∼56wt%), high MgO (∼8wt%), Mg# (61–67), Cr (174–330ppm) and Ni (173–242ppm) contents; low Y (∼13ppm) and Yb (∼1.2ppm) concentrations, relative to typical island arc andesites. They exhibit light-REE enriched (LaN/SmN∼1.7) and heavy-REE depleted (GdN/YbN∼1.9) fractionated REE patterns. On a primitive mantle normalized trace element variation diagram they exhibit negative Nb and Ti, and contrasting positive Zr anomalies relative to the neighboring REE. The La–Yb systematics suggest that these high-Mg andesites were generated by hydrous partial melting of a slab-melt metasomatized mantle wedge in the sub-arc mantle. The felsic volcanic rocks are characterized by high SiO2∼73wt%, Na2O∼4.5wt%, and significantly low concentrations of MgO (∼0.72wt%), Cr∼19ppm, Ni∼23ppm, Y∼5.9, and Yb∼0.44ppm; strongly fractionated chondrite normalized REE patterns (LaN/YbN∼30) with occasionally minor positive Eu anomaly, and primitive mantle normalized negative Nb and Ti, and contrasting positive Zr anomaly. The geochemical attributes of the Veligallu felsic volcanic rocks are analogous to the Cenozoic high silica-type adakites that are produced by partial melting in the sub-arc mantle, and presumably without any significant interaction of the slab-melts with the overlying mantle wedge.The juxtaposed occurrence of basalts with the banded iron formations, and their close spatial association with the high-Mg andesites, in the Veligallu greenstone belt, indicates that the petrogenesis of these volcanic rocks is consistent with their eruption in an intraoceanic setting. The close proximal association of the back-arc type basalts, and the high-Mg andesites and adakites within the Veligallu greenstone belt potentially suggests that this greenstone belt represents a rifted back-arc crust. This event appears to have significantly contributed to the peak of crustal growth activity in the eastern Dharwar craton at 2.7Ga.

2.2Ga magnesian andesites, Nb-enriched basalt-andesites, and adakitic rocks in the Lüliang Complex: Evidence for early Paleoproterozoic subduction in the North China Craton

The Lüliang Complex is located at the western margin of the middle segment of the Trans-North China Orogen, along which the Western and Eastern Blocks amalgamated to form the basement of the North China Craton. The complex consists of the Paleoproterozoic granitic plutons and meta-supracrustal rocks, of which the latter are subdivided into the Jiehekou, Lüliang, Yejishan, and Heichashan/Lanhe Groups. A meta-volcanic rock from the Yejishan Group gives a magmatic zircon U–Pb age of 2188 ± 48 Ma and εHf(t) values of + 0.06 to + 5.42. Based on the geochemical characteristics, four different ~ 2.2 Ga igneous rock suits have been identified in the Lüliang Complex. The calc-alkaline basalts and andesites show the geochemical features of typical ‘normal’ arc volcanics with pronounced LREE enrichments relative to HREE, negative Nb, Ta, P and Ti anomalies and εNd(t) values of − 2.69 to + 2.14. The adakitic rocks are characterized by high Na2O/K2O ratios, weak to positive Eu anomalies, positive Sr and Ba but negative Nb and Ti anomalies, and relatively clustered and near to zero εNd(t) values. The Nb-enriched basalts and andesites have higher Nb, Zr and TiO2 contents, higher Nb/Ta (14.4–19.3) and Nb/U (20.6–23.2) ratios than the majority of arc basalts and andesites and relatively variable εNd(t) values (− 1.54 to + 3.02). The magnesian andesites are characterized by anomalously high MgO, Cr and Ni contents and have εNd(t) values of − 3.90 to + 1.17. The occurrence of adakitic rocks, Nb-enriched basalts and andesites and magnesian andesites has been described from many modern arcs featuring subduction of oceanic slab. Therefore, we suggest that similar mechanism may have played an important role in the production of the ~ 2.2 Ga igneous rocks in the Lüliang Complex and thus the final collision between the Eastern and Western Blocks along the TNCO must have happened at some time after ~ 2.2 Ga.